Arthrodires 587 



" Arthrothoraci with head-shield hexagonal in outline. 

 Median occipital trapezoidal, margins underlapped conspicu- 

 ously by the external occipitals. Prefrontals meet below pineal 

 plates, thus occluding this element from contact with centrals. 

 The median dorsal plate elongated, terminating in an acute heavy 

 point; no definite ventral keel; its anterior border approaches 

 the head-shield more closely than in related families. Cranio- 

 dorsal joint relatively small. Post ero-dorso-lateral large." (?A 

 pair of spines occurs in the pectoral region.) The best-known 

 species is Coccosteus cuspidatus (decipiens] of the Lower Red 

 Sandstone or Devonian of Scotland. 



The family of Dinicktkyida consists of "Arthrothoraci with 

 stout trenchant jaws, whose cutting surfaces have worn away 

 marginal teeth. Plates heavy. Head-shield with conspicuous 

 lateral indentation to form dorsal border of orbit. Preorbitals 

 separated by rostral and pineal elements, the latter passing 

 backward between the anterior ends of the centrals. Cranio- 

 dorsal joint conspicuous. Median dorsal shovel-shaped, nearing 

 a stout keel with a large neck and with heavy gouge-shaped 

 terminal. Postero-dorso-lateral relatively small in size." Di- 

 nichthys hertzeri and numerous other species are described from 

 the Devonian and Carboniferous rocks of Ohio. 



The Titanichthyidcs are "Arthrothoraci with slender edentu- 

 lous jaws bearing a longitudinal sulcus. Plates squamous. 

 Head-shield wide, with indentations to form dorsal border of 

 orbit. Cranio-dorsal joint complete, but of relatively small size. 

 Median dorsal with lateral border indented with rudimentary 

 keel and with flat and rounded terminal. Antero-dorso-lateral 

 with an area of overlap on median border." Titanichthys agas- 

 sizi is a gigantic mailed fish from the Lower Carboniferous of 

 Cleveland, Ohio. 



The Mylostomidce are "Arthrothoraci with dental elements in 

 the character of crushing plates. Cranial shield wide, rounded 

 anteriorly, deeply indented in nuchal margin; orbital rim not 

 apparent in dorsal aspect. Central separated from marginal." 

 Mylostoma terrelli is based on jaws from Cleveland, Ohio. 



The Selenosteida are "Arthrothoraci with jaws studded with 

 cuspidate teeth ; the mandibular rami rounding out anteriorly or 

 presenting diverging tips, bearing teeth in the symphysis. Cranial 



